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A cross-linkable conjugated polymer with bromo groups on the side chain has been developed and used as an anode interlayer to improve the active layer morphology and consequently enhance the device performance of polymer solar cells (PSCs). The polymer, PCDTBT-Br, has cross-linkable bromo groups attached to the side chain of a widely-used donor polymer, poly[N-9-heptadecanyl-2,7-carbazole- alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT). The pendant bromo groups do not significantly change the LUMO/HOMO energy levels and absorption spectrum of the PCDTBT polymer backbone. PDCTBT-Br can crosslink under UV irradiation to give a robust film, which enables multilayer PSC device fabrication. Moreover, the much lower surface energy of PCDTBT-Br (20.4 mJ m−2) compared to PEDOT:PSS (91.6 mJ m−2) is beneficial for achieving optimal active layer morphology. As a result, with PCDTBT:[6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as the active layer, the PSC device with PCDTBT-Br as the underlying layer shows a power conversion efficiency (PCE) of 6.59 %, in comparison to a PCE of 5.86 % of the control device. The device performance enhancement is ascribed to the much improved phase separation with a fibrillar nanostructure in the active layer. [ABSTRACT FROM AUTHOR]